Morphological Studies on the Peripheral Small Arteries of Spontaneously Hypertensive Rats

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Morphological studies of the jejunal small arteries and arterioles of spontaneously hypertensive rats (OKAMOTO and AOKI) were performed by direct observation, and by histological, histometric, enzyme-histochemical and fluorescence methods. For making the histological preparations, quick freezing and freeze-substitution or freeze-drying methods were employed to preserve the functional stage of the blood vessels. Rats were divided into three groups: the pre-hypertensive or transitional stage (40 to 60 days after birth), the initial stage (four to six months after birth) and the advanced stage (over one year after birth) of hypertension, and the following results were obtained. 1) Segmental constrictions and tortuosities of the jejunal small arteries and arterioles were observed in all cases of spontaneously hypertensive rats after the development of hypertension. 2) Minimal luminal diameters of almost all branches of the jejunal small arteries and arterioles in spontaneously hypertensive rats were significantly smaller than in normotensive control rats, in the range of 64% to 85% of the control diameters . 3) The mean luminal diameters of the jeju-nal small arteries and arterioles in spontaneously hypertensive rats were significantly smaller in three-fourths of the branches, while the maximal luminal diameters were significantly smaller in half the branches, than in normotensive control rat s. 4) Adenosine triphosphatase activity of the jejunal arterial walls of spontaneously hypertensive rats showed an increase in half the cases in the pre-hypertensive or transitional stage and in all cases in the initial and advanced stages. 5) Adenosine monophosphatase activity of the jejunal arterial walls of spontaneously hypertensive rats showed no difference in all cases in the pre-hypertensive or transitional stage, an increase in two-thirds of the cases in the initial stage, and in all except one in the advanced stage. 6) Adrenergic nerve terminals around the jejunal small arteries and arterioles of spontaneously hypertensive rats were significantly larger than in normotensive control rats in the pre-hypertensive and initial stages, and showed no change in the advanced stage. 7) From the above-mentioned evidence, it may be concluded that increased activity of the sympathetic nervous system and increased contracting tendency of the vascular smooth muscle cells cause angiospasm, which plays an important role in the development and maintenance of hypertension.

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